1. Field of the Invention
The present invention relates to a position sensing system for a switched reluctance (SR) machine.
2. Description of the Related Art
Some known SR machines, such as Controlled Power Technologies' Speedstart®, incorporate a sensing mechanism comprising a sensor and a magnet. The stationary sensor senses changes in a magnetic field produced by the magnet, which is mounted on an end of the rotating shaft of the SR machine.
The sensor has a working range, or tolerance band, defined by an upper and a lower value of magnetic flux density. If the magnetic flux density is below the lower value of the tolerance band, the sensor cannot detect it and therefore cannot function correctly. If the magnetic flux density value exceeds the upper value of the tolerance band, the sensor signal becomes saturated.
FIG. 1 is a graphical example of magnetic decay in flux density vs distance in accordance with a prior art sensing system. As shown in this graph, magnetic flux density decreases in a non-linear manner as the sensor is moved away from the magnet. The magnetic field strength of a particular magnet, in combination with the working range of the sensor, dictates a function, or operating range band for the particular sensor/magnet combination.
Whilst phase winding connections can be configured to result in no electromagnetic field along the centerline of the machine (i.e. the position sensor axis) in practice a number of factors may result in a non-zero field. These factors include uneven current sharing in parallel conductors due to localized heating affecting resistivity, differing lengths of winding connections, and rotor eccentricity.
Known SR machines are formed of components of various materials such as steel, plastics and aluminum, having differing coefficients of thermal expansion and magnetic attenuation properties. Since typical operating temperatures of an SR machine could be in the range of −40° C. to 200° C., a significant degree of movement can occur between the sensor and the magnet as a result of thermal expansion of components. This expansion results in a change in the magnetic field strength sensed by the sensor, and at extreme temperatures may result in the magnetic flux density falling outside the working range of the sensor.
Furthermore, the frequent switching of high currents within the SR machine produces electromagnetic interference. Firing phase currents with a small angular position error can result in very high current spikes and unpredictable performance. This is especially true when coils are fired with the rotor is in an unaligned position without the inductance (or rising inductance) to limit the current level.
For this case, when the coils are fired in the unaligned position the stray fields are likely to be much greater since the stator teeth will provide flux into a large air gap and hence the fields will not be fully contained within the steel laminations.
Large currents with a relatively high production of stray fields will cause exacerbated sensor errors. Since the sensor is in the machine control loop it can be anticipated that relatively small position errors can ultimately lead to loss of control.
The present invention is aimed at providing a position sensing system for an SR machine wherein the working range of the sensor is maximized, and wherein the problems discussed above are at least mitigated.